A global positioning system (GPS) has been used in various fields such as a geodetic survey, a survey, a scientific observation and a time synchronization. A GPS signal has a 1575.42 MHz of L1 frequency and a 1227.6 MHz of L2 frequency. Such a GPS signal is delayed while passing the atmosphere due to an ionosphere error, a troposphere error, and a multipath error. These errors degrade the accuracy to determine an orbit in a navigation system or a precise geodetic survey. After releasing a S.A. effect at May 2005, it is possible to eliminate the ionosphere delay that is a main error factor.
The precise observation data can be received using the dual frequency GPS receiver. However, a single frequency is generally used for economic reason or when the dual frequency is not available. There are two representative methods of reducing the ionosphere error due to the single frequency. That is, these are a method of using directly observed data values and another method of using an ionosphere model.
The method of using the ionosphere model produces large error values when the elevation angle of GPS satellite is lowered. The method of using the ionosphere model generally produces inaccurate data compared to the method of using the observed data values. Such an inaccurate observation data degrades the accuracy of an orbit.
There are two methods in the method of using the observed data, which are a graphic method and a DRVID method.
The GRAPHIC method generates new data by averaging a pseudorange and carrier phase of the observation epoch. That is, an integer ambiguity must be precisely calculated to use for orbit determination. On the contrary, the DRVID method corrects ionosphere error in carrier phase or pseudorange of original data. The DRVID method must calculate two average data of pseudorange and carrier phase at two times, and calculated a difference of two average data. Herein, the DRVID method does not need to calculate an integer ambiguity when the signal is locked since the integer ambiguity is identical per each pass of the signal while the signal is locked. Inaccurate integer ambiguity degrades the precision of the observed data, and estimation of integer ambiguity is very annoying process in software view. On the contrary, the DRVID method must recognize ionosphere error value in a time of the shortest distance of a signal per pass, that is, when the elevation angle is highest. Also, the DRVID method must estimate a bias value per one pass to know the ionosphere error at a time of the highest elevation angle while determining a precise orbit.